1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system having the same and, more particularly, to a diffractive optical element of such a grating structure that diffracts light (energy) of a plurality of wavelengths or a certain band so that the diffracted light concentrates on a particular order (design order), and an optical system having the same.
2. Description of Related Art
One of the conventional methods of correcting the chromatic aberrations of the optical system is to combine a plurality of glasses (lenses) of different dispersions (Abbe numbers) from one another.
In addition to the above method of lessening the chromatic aberrations by using the combination of glass materials, there is another method of using a diffractive optical element having the diffracting function in the lens surface or the surface of other parts of the optical system, as disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-Open Patent Applications No. Hei 4-213421 and No. Hei 6-324262, and U.S. Pat. No. 5,044,706, etc.
This method is based on the physical phenomenon that, for the rays of light in the wavelengths other than a reference wavelength, the refractive surface and the diffractive surface in the optical system produce chromatic aberrations in opposite directions to each other.
Further, in such a diffractive optical element, when the period of its diffraction grating is made to vary depending on the place, the diffractive optical element can take an effect similar to an aspherical lens, giving a great advantage of reducing the aberrations of the optical system.
Here, on comparison of the refracting action of light, for the lens surface, one ray of light, even after being refracted, remains the one. For the diffraction grating, on the other hand, it is typical that one ray of light, when diffracted, is divided into a plurality of rays of light of different diffraction orders.
To employ the diffractive optical element in the lens system, therefore, determination of the grating structure must be made such that, for a useful wavelength region, the light ray diffracts in concentration on a particular one order (design order). In a case where the energy of incident light concentrates on the diffracted light of the particular order, the intensities of the diffracted light rays of the other orders become low. If the sum of the intensities of the diffracted light rays of the other orders is zero, the diffracted light rays of the other orders are considered to be not present.
To this purpose, it becomes necessary that, for the design order, the light ray diffracts with a high enough efficiency (ideally, 100%). It should be also noted that, if the diffracted light of any of other orders than the design order is present, it forms an image at a different place from that of the design order, becoming flare.
In the optical system that utilizes the diffractive optical element, therefore, it is important to fully consider not only the spectral distribution of the diffraction efficiency for the design order, but also the behavior of the diffracted light of the other orders.
Suppose, as shown in FIG. 1, when a diffractive optical element 1 is formed with a diffraction grating 3 in one layer on a substrate 2 or a surface in the optical system, then the diffraction efficiencies for particular orders are obtained as shown in FIG. 2. In the graph of FIG. 2, the abscissa represents the wavelength, and the ordinate represents the diffraction efficiency. This diffractive optical element is so designed that, for the diffracted light of the first order (shown by a solid line curve), the diffraction efficiency becomes highest in the useful wavelength region.
That is, the design order is the first order. In addition, there are also shown the diffraction efficiencies for diffraction orders near the design order, i.e., or zero order and second order ((1xc2x11)st orders).
As shown in FIG. 2, in the design order, the diffraction efficiency has a highest value at a certain wavelength (540 nm) (hereinafter, referred to as the xe2x80x9cdesign wavelengthxe2x80x9d), and gradually lowers as the wavelength goes away from the design wavelength. This lowering of the diffraction efficiency in the design order is reflected to the diffracted light of the other orders, thereby producing flare. Also, in a case where a plurality of diffractive optical elements are in use, it particularly results that the diffraction efficiency lowers in the wavelengths other than the design wavelength. This leads to a decreases in the transmittance of the entire optical system.
An arrangement for reducing this lowering of the diffraction efficiency is proposed in U.S. patent application Ser. No. 09/121,685 (Japanese Patent Application No. Hei 9-217103). FIG. 3 is a sectional view of the main parts of the diffractive optical element 1 proposed in U.S. patent application Ser. No. 09/121,685. The diffractive optical element 1 shown in FIG. 3 has a laminated cross-section form with two layers 4 and 5 of diffraction gratings on a substrate 2 in superimposed relation to each other. Then, the refractive indices and dispersion characteristics of the materials of the two layers 4 and 5 and their grating thicknesses are optimized to obtain higher diffraction efficiencies throughout the entire range of useful wavelengths.
In the type of diffractive optical element shown in FIG. 3, as the material of the diffraction grating for each layer, use may be made of easy-to-cut optical glasses, plastics, or optically transparent, ultraviolet curable polymer. In this case, however, it becomes difficult to take as large a difference in the refractive index as in the mono-layer type. Therefore, the large difference in the optical path length becomes harder to take. For this reason, the diffraction grating becomes considerably thick. For example, in the diffractive optical element 1 of the two-layer structure, the material used for the first layer 4 is assumed to be an ultraviolet curable polymer of refractive index nd=1.525 and Abbe number xcexdd=47.8, and the material used for the second layer 5 is assumed to be another ultraviolet curable polymer of refractive index nd=1.635 and Abbe number xcexdd=23.0. In this combination, the grating thicknesses are optimized. Then, the resultant diffraction efficiency is shown in FIG. 4. It is understandable that the diffraction efficiency of the first order is kept high over the entire visible spectrum. In this case, however, the first diffraction grating 4 has a thickness d1 of 12.70 xcexcm, and the second diffraction grating 5 has a thickness d2 of 9.55 xcexcm. On consideration of the usual one-layer diffraction grating whose thickness is about 1 xcexcm, the two-layer diffraction grating has so much a large thickness. Also, in actual practice of manufacturing, because the second layer 5 shown in FIG. 3 is sectioned by every grating pitch, the use of the production technique by molding or the like results in a difficulty of transferring the form and detaching from the die.
An object of the present invention is to provide a diffractive optical element which is actually more practical to utilize than was heretofore possible. This utilizable diffractive optical element has such a fundamental structure that, as shown in FIG. 5, diffraction gratings 4 and 5 which differ in dispersion from each other are first formed in separation, then, both the diffraction gratings 4 and 5, while keeping their corresponding pitches to each other in alignment, are brought into a near juxtaposition, and through a certain space whose refractive index is xe2x80x9c1xe2x80x9d (for example, air), the diffraction gratings 4 and 5 are superimposed on each other.
Such diffraction gratings are manufactured by the ruling machine. So, the product can be used directly as the actual optical element. It may otherwise be used as a master grating, from which to produce replica gratings. In the former case, as the edge angle of the diffraction grating is more acute than that in the conventional or mono-layer type. If, as the diffractive optical element is made directly by ruling, the material is plastic or the like, there is a high possibility of chipping off the edges during ruling. In the latter case, when detaching the cast from the mold, the tip of the edge becomes dull or like phenomenon occurs, because the edge angle is so much acute.
Now, for the structure of the diffractive optical element as shown in FIG. 5, with the use of the brittle material described above, a form is considered that the first and second diffraction gratings 4 and 5 have their grating edges cut by 0.5 xcexcm from the tip, as shown in FIG. 6. The diffraction efficiency obtained in this situation is shown in FIG. 7. In calculation, the grating pitch used is 70 xcexcm. From the graph of FIG. 7, it is understood that the diffraction efficiency has lowered 3.5% or so over almost the entire visible spectrum. This lowering is reflected to the production of flare. In application to the diffractive optical element that has a pair of confronted diffraction gratings made of materials which differ in dispersion from each other, therefore, the diffraction efficiency for a particular diffraction order (design order) must be raised over the entire range of useful wavelengths. For this purpose, at least part of tips of a grating surface of one of the diffraction gratings and at least part of valleys, corresponding to the tips, of a grating surface of the other of the diffraction gratings are chamfered to a predetermined shape or formed to the predetermined shape (chamfered shape). Thus, the amount of flare can be reduced. In the following, including the claims, what is called the xe2x80x9cchamferedxe2x80x9d shape in the present specification includes the shape obtained by chamfering and the shape obtained by forming to the predetermined shape.
In accordance with an aspect of the invention, there is provided a diffractive optical element, which comprises a pair of diffraction gratings, the pair of diffraction gratings differing in dispersion from each other, the pair of diffraction gratings confronting each other through a space of a refractive index of 1, wherein a maximum optical path length difference occurring in the pair of diffraction gratings with respect to each of at least two wavelengths is integer times the associated wavelength.
In accordance with another aspect of the invention, there is provided a diffractive optical element, which comprises a pair of diffraction gratings, the pair of diffraction gratings differing in dispersion from each other, the pair of diffraction gratings confronting each other through a space of a refractive index of 1, wherein a maximum optical path length difference occurring in the pair of diffraction gratings with respect to each of at least two wavelengths is integer times the associated wavelength, and peak portions and valley portions of the pair of diffraction gratings are chamfered or formed in a chamfered shape.
In accordance with a further aspect of the invention, there is provided a diffractive optical element, which comprises a pair of diffraction gratings, the pair of diffraction gratings differing in dispersion from each other, wherein a maximum optical path length difference occurring in the pair of diffraction gratings with respect to each of at least two wavelengths is integer times the associated wavelength, and peak portions and valley portions of the pair of diffraction gratings are chamfered or formed in a chamfered shape.
In accordance with a further aspect of the invention, there is provided a diffractive optical element, which comprises a substrate, and a diffraction grating formed on the substrate, wherein either or both of peak portions and valley portions of the diffraction grating are chamfered or formed in a chamfered shape.
In the pair of diffraction gratings described above, there are embodiments, one of which is to divide the entire ruled surface into a plurality of zones, wherein the size and/or form of the chamfered portions is or are different with the different zones, and the other of which is not to differentiate this size or form.
A further embodiment is that the chamfered area has a form of a flat plane and, as the flat plane is projected onto the surface of the substrate on which the diffraction grating is formed, the length xe2x80x9caxe2x80x9d of the flat plane in a direction of grating arrangement of a grating surface lies within the following range:
0.5 xcexcm less than a less than 2 xcexcm 
Another embodiment is that the chamfered area has a form of a curved surface and, as the curved surface is projected onto a flat plane made by a direction (line) of grating arrangement of a grating surface and a normal line of the substrate on which the diffraction grating is formed, a radius of curvature xe2x80x9crxe2x80x9d of the curved surface lies within the following range:
0.5 xcexcm less than r less than 2 xcexcm 
An optical system according to the invention has a feature of using any one of the diffractive optical elements of the forms described above. As the optical system, mention may be made of an image forming optical system and an observation optical system.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated is characterized in that the Abbe number of at least one of the plurality of diffraction gratings is not more than 30.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that the Abbe number of at least one of the plurality of diffraction gratings is not more than 30.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 10 xcexcm and that the Abbe number of at least one of the plurality of diffraction gratings is not more than 30.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 10 xcexcm and that the Abbe number of at least one of the plurality of diffraction gratings is not more than 30.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that at least one of the plurality of diffraction gratings has an Abbe number of not less than 40 and is made from, for example, glass.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described and is characterized in that one of the plurality of diffraction gratings which has an Abbe number of not more than 30 is made from ultraviolet curable polymer.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 7.5 xcexcm and that the Abbe number of at least one of the plurality of diffraction gratings is not more than 25.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a high-molecular polymer is used for a material of at least one of the plurality of diffraction gratings and that a material having an Abbe number of not more than 25 is used for a material of at least another one of the plurality of diffraction gratings.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 10 xcexcm, that a high-molecular polymer is used for a material of at least one of the plurality of diffraction gratings, and that a material having an Abbe number of not more than 25 is used for a material of at least another one of the plurality of diffraction gratings.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 10 xcexcm, that a high-molecular polymer is used for a material of at least one of the plurality of diffraction gratings and that an ultraviolet curable polymer having an Abbe number of not more than 25 is used for a material of at least another one of the plurality of diffraction gratings.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 7.5 xcexcm, that a high-molecular polymer is used for a material of at least one of the plurality of diffraction gratings, and that a material having an Abbe number of not more than 20 is used for a material of at least another one of the plurality of diffraction gratings.
In accordance with a further aspect of the invention, a diffractive optical element in which a plurality of diffraction gratings of respective different Abbe numbers are laminated to such a grating structure as to heighten the diffraction efficiency of diffracted light of a particular order throughout an entire usable wavelength region is characterized in that a grating thickness of each of the plurality of diffraction gratings is not more than 7.5 xcexcm, that a high-molecular polymer is used for a material of at least one of the plurality of diffraction gratings and that an ultraviolet curable polymer having an Abbe number of not more than 20 is used for a material of at least another one of the plurality of diffraction gratings.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that the plurality of diffraction gratings include at least one diffraction grating which differs from the others in grating direction.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that the usable wavelength region is a visible spectrum.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that the plurality of diffraction gratings are formed on a transparent substrate and that, among the plurality of diffraction gratings, a diffraction grating nearest to the transparent substrate is made from the same material as that of the transparent substrate.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that each of the plurality of diffraction gratings satisfies a condition of xe2x80x9cd/P less than ⅙xe2x80x9d, where P is a grating pitch thereof and d is a grating thickness thereof.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that each of the plurality of diffraction gratings satisfies a condition of xe2x80x9c1 less than d less than 6xe2x80x9d, where d is a grating thickness (xcexcm) thereof.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that the diffractive optical element is designed such that the diffraction efficiency thereof becomes 97% or higher throughout the entire usable wavelength region.
In accordance with a further aspect of the invention, a diffractive optical element has any one of the forms of the elements described above and is characterized in that the diffractive optical element is designed such that, with respect to each of spectral d-line, F-line and C-line, the diffraction efficiency thereof becomes 99% or higher.
In the diffractive optical element according to any one aspect of the invention, in a case where there are three layers of diffraction gratings, the number of kinds of materials for the diffraction gratings is not confined to be equal to the number of layers. Thus, the number of kinds of materials for the diffraction gratings may be made smaller than the number of layers. For example, in the case of the 3-layer type, it is also possible to make an arrangement that the diffraction gratings in the first and third layers are of the same material, while a different material from the above material is used for the diffraction grating in the second layer.
In the diffractive optical element according to any one aspect of the invention, it is preferred that any adjacent two of the diffraction gratings are laminated through an air layer.
An optical system of the invention is characterized by having any one of the diffractive optical elements described above.
An image forming optical system of the invention is characterized by having any one of the diffractive optical elements described above.
A photographic optical system of the invention is characterized by having any one of the diffractive optical elements described above.
An observation optical system of the invention is characterized by having any one of the diffractive optical elements described above.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.